In Magnetoresistive Random Access Memory (MRAM), data is stored by magnetic storage elements. The elements are formed from two ferromagnetic layers, each of which can hold a magnetic field, separated by a thin tunnel barrier layer. One of the two layers is a permanent magnetic layer set to a particular polarity; the other's field will change to match that of an external field. A memory device is built from a grid of such cells.
Reading is accomplished by measuring the electrical resistance of the cell. A particular cell is typically selected by powering an associated transistor that switches current from a supply line through the cell to ground. Due to the magnetic tunnel effect, the electrical resistance of the cell changes due to the orientation of the moments in the two magnetic layers. By measuring the resulting current, the resistance inside any particular cell can be determined, and from this the polarity of the writable layer. Typically if the two layers have the same polarity this is considered to mean “0”, while if the two layers are of opposite polarity the resistance will be higher and this means “1”.
Magnetic tunnel junctions (MTJs), a component in MRAM, include two ferromagnets (magnetic layers) separated by a thin tunnel barrier layer (dielectric tunneling layer) as described above. MTJs are manufactured using thin film technology and prepared by photolithography. The first magnetic layer is referred to as a fixed/pinned layer because its magnetic polarity is fixed, while the second layer is referred to as a free layer because its magnetic polarity changes when the magnetic cell is written to. On top of the free layer, typically a conductive top conducting layer provides thermal and magnetic stability for the MTJs.
For MRAM processes, especially 65 nm and/or 45 nm process and beyond, an important issue is the oxidation of the MTJ free layer (i.e. a dead layer) that impacts the free layer performance and the etching uniformity. The free layer is oxidized during the photoresist ashing process. Ashing process is the removal of organic materials, e.g. photoresist, by volatilization from the wafer surface using strongly oxidizing ambient; e.g. oxygen plasma ashing. However, O2 ashing will also oxidize the free layer.
Another issue is the total resistance through the top conducting layer and free layer, because with increased resistance, the writing voltage needs to be also increased. This situation is not desirable for MRAM, and especially not suitable for spin-torque-transfer (STT)-MRAM for example.
Accordingly, new methods to decrease the dead layer in MTJ free layer, not increase the total resistance of MTJ, and increase the yield of MRAM cell are desired.